An optical sensor for recording the wetting of a surface, particularly the wetting of a motor vehicle window, is described in German Published Patent Application No. 199 43 887 having a transmitter and a receiver for electromagnetic waves, the surface being present in a sensing region between the transmitter and the receiver, so that the formation of a wetting on the surface effects a change in the signal detected by the receiver. Furthermore, in that location the optical sensor has a light guiding element by which the electromagnetic waves are bidirectionally guided into the sensing region or guided away from the sensing region. Finally, it is known from German Published Patent Application No. 199 43 887 that one may position a retroreflector, particularly a holographic retroreflector, in such a way that it feeds back electromagnetic waves reflected by the surface to the surface, and from there to the light guiding element. The optical sensor thus described may be used especially as a rain sensor, the light coupled in when the window is dry being totally reflected at the window's outer surface, whereas, when the window is wet, the total reflection is interrupted, which is what effects the detected signal change.
Another rain sensor is described in European Published Patent Application No. 0 999 104 in which a holographic film is situated between sensor and receiver.